1. Field of the Invention
The invention relates to the use of biological or chemical compounds for the detection of the presence of other biological or chemical compounds within a specimen, and more particularly, to a method and apparatus for "spotting" the differing compounds onto a test array.
2. Description of Related Art
Biological or chemical compounds (commonly referred to as "probes") are commonly used as reagents in the detection of other target biological or chemical compounds, such as certain viruses or bacteria, within a specimen under test. Any such target compounds existing within the specimen can be identified though the controlled exposure of the specimen to the probe and the detection of DNA hybridization or antibody-antigen reactions. For example, the binding between an antibody and molecules displaying a particular antigenic group on their surface may be used as a basis for detecting the presence of the antibody, molecules carrying the antigenic group, or the antigenic group itself. Distinct varieties of reagent probes can be specifically formulated to detect particular target compounds. Accordingly, the specimen can be evaluated for the presence of a wide assortment of target compounds by exposure to a variety of probe types.
In particular, the probe may contain biological material having target DNA of up to a thousand base pair in length. Upon controlled exposure of a specimen having a biological match with the target DNA, the specimen and target DNA hybridize, or bind together. The presence of the target DNA within the specimen is detected by evaluation of whether the hybridization has occurred. The target DNA can be labelled with fluorescent tags that can be detected by exposure to particular wavelengths of light, such as ultra-violet light. Optical detection of the fluorescent emission from the probe indicates that the specimen has hybridized with the probe.
To prepare a test using a particular probe, a small quantity of the probe in solution is dispensed onto a prepared glass slide or other test surface, a process referred to as "spotting." The specimen is then exposed to the spotted probe and the specimen permitted to selectively hybridize with the probe. After excess specimen material has been rinsed off the test surface, any hybridization with the probe can be readily detected. Probe tests of this nature represent a relatively simple and cost effective method for clinically evaluating a specimen.
A drawback of the conventional probe test method is that it can be time consuming and cumbersome in situations in which the detection of numerous target compounds is desired. Typically, it is desirable to evaluate a specimen for the presence of a wide assortment of target biological and/or chemical compounds. In these situations, separate probe tests for each target compound would have to be conducted. To avoid this unnecessary duplication of effort and expense, a possible solution would be to utilize a single test slide that is pre-spotted with an assortment of probes, and to expose the specimen to all the probes simultaneously. This way, reactions of each of the probes to the specimen can be identified during a single procedure.
This solution is severely limited, however, by practical aspects of spotting the probe onto the slide. To prepare a test slide pre-spotted with a large number of probes, each of the probes must necessarily have a rather minuscule volume, such as on the order of one nanoliter. Spaces should be provided between adjacent ones of the probes to prevent undesired mixing of the probes. An additional consideration is that the amount of specimen material that ultimately hybridizes with the probes at the particular spots depends, in part, on the volume of the probe dispensed at the spot. Thus, the volume of the probe must be precisely controlled.
A secondary problem with conventional probe spotting is the control of probe placement on the test slide. The positional accuracy of the placement of the probes onto the test slide is critical to accurate correlation of detected hybridization with a particular probe. Due to the extremely small volume of the probe dispensed onto the test slide, the placement accuracy should be within the sub-millimeter range.
Another drawback with conventional probe spotting that relates to the probe placement problem is the difficulty in preventing false negatives. Due to the relatively low accuracy of conventional spotting techniques, a probe spot can be placed in an incorrect position on a test slide, or a probe can be inadvertently omitted from the test slide altogether. In a test of a specimen in which a particular probe fails to hybridize with a specimen, the normal conclusion to be drawn is that the specimen is negative with respect to the existence of a particular target compound. This conclusion would be erroneous, however, if the probe had not been properly spotted onto the test slide.
The accurate dispensing of small volumes of liquid has been previously demonstrated in conventional ink-jet printers. The cartridges utilized in ink-jet printers dispense a controlled volume of ink onto a paper substrate by use of a pressure wave created within the cartridge. Nevertheless, this approach would not be applicable to probe spotting, since the pressure wave results from a sharp temperature increase to the ink that would otherwise damage or sheer the probe due to the inherent fragility of the probe. Further, the ink-jet cartridges produce a high degree of splattering that is acceptable for printing applications, but would risk contamination of adjacent probe spots in this application. Thus, the conventional ink-jet technology would not be a feasible method for measuring and controlling the accurate placement of probes of such small fluid volumes.
Therefore, a critical need exists for a method and apparatus for accurately spotting minuscule volumes of biological probes in solution onto a test slide. Such a method should provide for precise control over probe volume, as well as placement. The method should also be cost effective and readily adaptable for large scale production of test slides having relatively high numbers of individual probes.